The Audit Committee was, at first, quite small and comprised mostly of Board members. But we found that fairly few trustees from the community had both the experience and the time to focus on its work. So for the 2009-2010 year we switched to a model where one trustee leads the Audit Committee (me), and then we reach out broadly to the community for members. We've had great success with that model, and continue to have really valuable participation from community members. Membership history is at
http://wikimediafoundation.org/wiki/Audit_committee. The Board's Chair and the ED sit in on meetings.